Composite materials are used in propulsive blade systems in aircraft and confer a number of benefits, including weight reduction and corresponding fuel efficiency. In systems that incorporate a blade variable-pitch feature, the base portions or root portions of propulsor blades typically terminate in a cylindrical shape to accommodate a variety of low friction bearing assemblies that generally include ball and/or roller elements. These bearing assemblies interface with individual blade sockets or “arms” of a central hub assembly that retains the blades and allow each blade to be rotatable about the bearing axis. A variable-pitch propulsive blade system includes a pitch control system that allows the pitch angle of all blades to be varied simultaneously to maximize thrust for different aircraft operating conditions, resulting in greater aircraft propulsive efficiency. In some VTOL (Vertical Take-Off and Landing) aircraft applications, such as helicopters, the pitch angle of each blade in multiple-bladed lift rotors can be varied in a cyclic manner to affect the direction of thrust for the purpose of controlling the direction of flight. This method of blade pitch change is referred to as cyclic-pitch control.
An example of a propulsive blade assembly designed to reduce weight is discussed in U.S. Pat. No. 6,676,080, issued Jan. 13, 2004, entitled “Composite Airfoil Assembly,” which is incorporated herein in its entirety.
However, because outer airfoil portions of composite blades are typically very lightweight, there is significantly less centrifugal load developing at the blade root portion than the centrifugal force that develops with older, solid or hollow metal blades, or blades with metal spars covered with composite shells. When root bending loads are high, the centrifugal load at the root joint of a blade may be insufficient to ensure the integrity of the connection between the blade and a hub assembly or other support mechanism, especially in cases where blade vibratory excitations are high and blade response results in increased bending loads that can adversely affect the composite-to-metal root joint within the blade root portion that interfaces with the hub assembly.